Viruses in which a part of the genome is missing are called defective viruses. The Bryan strain of RSV appears to lack envelope glycoprotein and does not form infectious progeny. If cells infected with RSV are super-infected with a leukemia virus, the RSV becomes infectious. The leukemia virus codes envelope glycoprotein and is known as a helper virus. The RSV viruses bearing leukemia virus envelopes are known as pseudo types. The helper virus genetically complements the RSV. A helper virus complements both these defects. Sometimes the helper virus associated with the defective virus may be replaced by another helper. This causes a change in the serological classification of the virus complex, since some or all the antigens of the virus particles are provided by the helper virus.



Transmission of oncornaviruses. Transmission of the virus may be horizontal or vertical. Horizontal transmission takes place from one host to another by contagion. Vertical transmission takes place from parent to offspring. Vertical transmission may be congenital or genetic. Congenital transmission is from the mother to the offspring through the ovum, placentra or milk.

Some viruses are transmitted only vertically. Such viruses are not very harmful to the host, because neither the host nor the virus would survive if they were. Other viruses are typically transmitted horizontally, but may be incidently transmitted vertically. Some viruses are horizontally transmitted in one host and vertically in another in mosquito borne yellow fever virus and tick-borne encephalitis. These viruses are pathogenic in the mammalian hosts but usually harmless in the arthropod hosts. The C-type viruses are transmitted by all the three methods, horizontal, congenital and genetic.